A protein has been synthesized in E. coli that contains HTLV-I gag gene sequences. The HTLV-I gag gene was placed into the mos gene coding sequences in the expression vector, pA28, a derivative of pJL6 developed in our laboratory. A 30 kDa protein can be detected by anti-mos peptide antibody. By use of an inclusion body purification protocol, this protein can be made 50% pure without the use of column chromatography. This protein is potentially useful as a diagnostic reagent since it is recognized by antibodies in patient serum. A protein was identified in nuclear extracts of the HTLV-I cell line, C10/MJ, that specifically binds a region on the LTR near the polyadenylation site. The specificity of binding was verified by demonstrating that it could be abolished by the addition of unlabeled LTR DNA as a competitor, but not by pBR322 DNA. Little or none of this activity was found in other cell lines tested, including MJ leukemic T-cells, indicating that this phenomena is specific to the C10/MJ line. DNA sequences from the sea urchin, Lytechinas variegatus, related to the v-ets oncogene from avian erythroblastosis virus, E26, were molecularly cloned. They were shown to have a high degree of sequence homology with the region of v-ets that is also homologous with the Hu-ets-2 domain found on human chromosome 21. Northern blot analysis of sea urchin tissues and developing embryos indicated that the gene is actively transcribed in the early stages of embryonic development and somewhat less so in unfertilized eggs. No transcript was detected in adult somatic tissues.